(a) Technical Field
The present disclosure relates to a gas diffusion layer (GDL) for fuel cell applications, which functions to discharge water as a product of an electrochemical reaction in a fuel cell stack and transfer electrons.
(b) Background Art
In general, a polymer electrolyte membrane fuel cell (PEMFC) is used as a fuel cell for a vehicle. The PEMFC should be able to stably operate over a wide current density range such that it normally exhibits a high-power performance of at least several tens of kW under various operational conditions of the vehicle [S. Park, J. Lee, and B. N. Popov, J. Power Sources, 177, 457 (2008)].
The fuel cell generates electricity through an electrochemical reaction between hydrogen and oxygen. Hydrogen supplied to an anode as an oxidation electrode of the fuel cell is dissociated into hydrogen ions and electrons. The hydrogen ions are transmitted to a cathode as a reduction electrode through a polymer electrolyte membrane, and the electrons are transmitted to the cathode through an external circuit. At the cathode, the hydrogen ions and electrons react with oxygen containing air to generate electricity and heat and, at the same time, produce water as a reaction by-product.
When an appropriate amount of water produced during the electrochemical reaction is present in the fuel cell, it performs the function of maintaining the humidity of a membrane electrode assembly (100). However, when an excessive amount of water is present and is not appropriately removed, a flooding phenomenon occurs at high current density, and the flooding water prevents the reactant gases from being efficiently supplied to the fuel cell, which results in an increase in the voltage loss.
Here, the functions of the gas diffusion layer included in the fuel cell will be described in more detail.
FIG. 1 is a schematic diagram showing the structure of a unit cell including gas diffusion layers.
The gas diffusion layer is attached to the outer surface of each of catalyst layers coated on both sides of a polymer electrolyte membrane of the unit cell for an oxidation electrode and a reduction electrode. The gas diffusion layers function to supply reactant gases such as hydrogen and air (oxygen), transfer electrons produced by the electrochemical reaction, and discharge water produced by the reaction to minimize the flooding phenomenon in the fuel cell.
Typically, a commercially available gas diffusion layer has a dual layer structure including a microporous layer (MPL) having a pore size of less than 1 μm when measured by mercury intrusion and a macroporous substrate (or backing) having a pore size of 1 to 300 μm [X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X. Zhong, Y. M. Liang, B. L. Yi, Electrochimica Acta, 51, 4909 (2006)].
The microporous layer of the gas diffusion layer is formed by mixing carbon powder such as acetylene black carbon and black pearl carbon with a hydrophobic agent such as polytetrafluoroethylene (PTFE) and coating the mixture on one or both sides of the macroporous substrate.
Meanwhile, the macroporous substrate of the gas diffusion layer is generally composed of carbon fiber and a hydrophobic agent such as PTFE and may be formed of carbon fiber cloth, carbon fiber felt, or carbon fiber paper [S. Escribano, J. Blachot, J. Etheve, A. Morin, R. Mosdale, J. Power Sources, 156, 8 (2006); M. F. Mathias, J. Roth, J. Fleming, and W. Lehnert, Handbook of Fuel Cells-Fundamentals, Technology and Applications, Vol. 3, Ch. 42, John Wiley & Sons (2003)].
It is necessary to optimize the structural design of the gas diffusion layer for fuel cell applications such that the gas diffusion layer provides appropriate performance according to its application fields and operational conditions. In general, in the formation of the gas diffusion layer for fuel cell applications, the carbon fiber felt or carbon fiber paper is preferred to the carbon fiber cloth since the carbon fiber felt and carbon fiber paper have excellent properties such as reactant gas supply properties, product water discharge properties, compression properties, and handling properties.
Moreover, the gas diffusion layer has a significant effect on the performance of the fuel cell according to complex and various structural differences such as the thickness, gas permeability, compressibility, degree of hydrophobicity, structure of carbon fiber, porosity/pore distribution, pore tortuosity, electrical resistance, and bending stiffness. Especially, it is known that there is a significant difference in the performance in the mass transport region (Japanese Patent No. 3331703 B2).
Recently, with the commercialization of the fuel cell, extensive research and development for the mass production of the gas diffusion layer as a core component of the fuel cell have continued to progress. The gas diffusion layer should provide excellent performance in the fuel cell and should have an appropriate level of stiffness to provide excellent handling properties when several hundreds of cells are assembled in the fuel cell stack. When the stiffness of the gas diffusion layer is very high in the roll direction of the GDL material, it is difficult to roll the GDL material for transport and storage, and thus the mass productivity is reduced. Moreover, according to the previous reports, when the stiffness of the gas diffusion layer is insufficient in the fuel cell, as shown in FIG. 2, the gas diffusion layer may intrude into flow field channels of a bipolar plate (or separator) during assembly of the fuel cell (which is called “GDL intrusion”) [Mao Nitta, Tero Hottinen, Olli Himanen, Mikko Mikkola, J. Power Sources, 171, 26 (2007); Yeh-Hung Lai, Pinkhas A. Rapaport, Chunxin Ji, Vinod Kumar, J. Power Sources, 184, 120 (2008); J. Kleemann, F. Finsterwalder, W. Tillmetz, J. Power Sources, 190, 92 (2009); M. F. Mathias, J. Roth, M. K. Budinski, U.S. Pat. No. 7,455,928 B2; T. Kawashima, T. Osumi, M. Teranishi, T. Sukawa, US 2008/0113243 A1].
When the GDL intrusion into the flow field channels of the bipolar plate (200) occurs, the space required for transferring reactant gases and product water is reduced, and the contact resistance between the gas diffusion layer (106), the ribs or lands (204) of the bipolar plate, and the polymer electrolyte membrane electrode assembly (100) is increased, which causes a significant deterioration in the fuel cell performance.
Since the GDL intrusion phenomenon is closely related with the flow field structure of the bipolar plate, it is important to appropriately design the flow field structure and increase the mechanical properties of the gas diffusion layer such as bending stiffness so as to achieve excellent fuel cell performance.
Typically, the fuel cell bipolar plate is composed of a major flow field and a minor flow field, and it is necessary to prevent the gas diffusion layer from intruding into the channels in the major flow field direction. For this purpose, it is important to increase the stiffness of the gas diffusion layer oriented in the width (W) direction rather than the length (L) direction which is in parallel with the major flow field direction of the bipolar plate. Otherwise, when the gas diffusion layer having a low stiffness is oriented in the width direction of the major flow field of the bipolar plate as shown in FIG. 2, the GDL intrusion into the major flow field of the bipolar plate is increased.
In order to solve this phenomenon, it is possible to use the inherent anisotropic properties of the gas diffusion layer.
That is, in the gas diffusion layer formed of carbon fiber felt or carbon fiber paper as a support, a greater amount of carbon fibers is oriented in the machine direction during the formation, and thus the gas diffusion layer in the machine direction has mechanical properties such as bending stiffness, tensile stress, etc. higher than those in the cross-machine direction (CMD) or transverse direction (TD).
Therefore, it is typical that the machine direction of the rolled GDL material is directed to the high stiffness direction and the cross-machine direction is directed to the low stiffness direction.
Conventionally, the gas diffusion layer is produced by intentionally arranging carbon fibers having a greater length or diameter in the cross-machine direction through a specific process or by introducing a metal reinforcing material to increase the stiffness of the gas diffusion layer in the width direction of the major flow field of the bipolar plate, thus preventing the gas diffusion layer from intruding into the channels (202) of the bipolar plate. Moreover, the gas diffusion layer is produced by arranging carbon fibers having a smaller length or diameter in the machine direction to facilitate the rolling of the GDL material to achieve the flexibility required for the rolling [M. F. Mathias, J. Roth, M. K. Budinski, U.S. Pat. No. 7,455,928 B2].
However, this method has problems that it is necessary to modify the method by adding a complicated process to the typical method for manufacturing the gas diffusion layer and, especially, when a different kind of metal reinforcing material is introduced, it may cause a variety of problems such as poor miscibility with the gas diffusion layer, non-uniform quality, etc.
According to another prior art method for preventing the GDL intrusion using anisotropic properties of carbon fiber woven cloth, the physical properties and handling properties of the cloth are insufficient, and thus it is difficult to use this method to manufacture the gas diffusion layer for fuel cell applications. [T. Kawashima, T. Osumi, M. Teranishi, T. Sukawa, US 2008/0113243 A1].
Accordingly, the previously proposed methods for preventing the GDL intrusion into the flow field channels of the bipolar plate are generally disadvantageous in terms of mass productivity, which is required for the commercialization of fuel cell vehicles.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.